Ultrafiltration and microporous membranes are used in pressure-driven filtration processes. Practitioners in the field of separation processes by membranes easily differentiate between microporous and ultrafiltration membranes and generally distinguish between them based on their application and aspects of their structure. Microporous and ultrafiltration membranes are made, sold and used as separate and distinct products. Despite some overlap in nomenclature, they are separate entities, and treated as such in the commercial world.
Ultrafiltration membranes are primarily used to concentrate or diafilter soluble macromolecules such as proteins, DNA, starches and natural or synthetic polymers. In the majority of uses, ultrafiltration is accomplished in the tangential flow filtration (TFF) mode, where the feed liquid is passed across the membrane surface and those molecules smaller than the pore size of the membrane pass through (filtrate) and the rest (retentate) remains on the first side of the membrane. As fluid also passes through there is a need to recycle or add to the retentate flow in order to maintain an efficient TFF operation. One advantage of using a TFF approach is that as the fluid constantly sweeps across the face of the membrane it tends to reduce fouling and polarization of the solutes at and near the membrane surface leading to longer life of the membrane.
Microporous membranes are primarily used to remove particles, such as solids, bacteria, and gels, from a liquid or gas stream in dead-end filtration mode. Dead-end filtration refers to filtration where the entire fluid stream being filtered goes through the filter with no recycle or retentate flow. Whatever material doesn't pass through the filter is left on its upper surface.
Ultrafiltration membranes are generally skinned asymmetric membranes, made for the most part on a support which remains a permanent part of the membrane structure. The support can be a non-woven or woven fabric, or a preformed membrane.
Microporous membranes are produced in supported or unsupported form. Usually, the support has the membrane or a portion of the membrane formed in the support, rather than on the support, as in ultrafiltration membranes.
The early cellulosic, nylon and polyvinylidene fluoride microporous membranes were symmetric and for the most part, unskinned. Presently, some asymmetric microporous membranes are produced, and some of these are skinned.
While it would seem that the two types of membrane could be differentiated by pore size, this is not the case, as will be discussed below. The reasons for this are that they are used in different applications, requiring different characterization methods. None of the methods usually used give an absolute pore size measure, and different methods cannot be directly compared.
Despite the similarities between microporous membranes and ultrafiltration membranes, the history of their development is quite different. It is therefore not surprising that there is more than one accepted demarcation between them.
Microporous membranes were commercially developed from the work of Zsigmondy by Sartorius Werke (Germany) in 1929. These were what are now call “air cast” membranes made by evaporating a thin layer of a polymer solution in a humid atmosphere. These membranes were and still are symmetric and generally unskinned. Since they were used to remove or hold bacteria, they were rated by the bacteria size that would be retained. This method resulted in pore size ratings in microns.
A common method used to rate microporous membranes is the bubble point test. In this method, the microporous membrane is placed in a holder and saturated with a test liquid. Gas pressure is applied to one side of the membrane and the pressure is increased at a fixed rate. The appearance of the first stream of bubbles from the downstream side is a measure of the largest pore. At a higher pressure where the liquid is forced out of the majority of the pores, the foam all over point (FAOP) is reached. These are described in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976).
Ultrafiltration membranes (UF) are a spin-off of the reverse osmosis membrane development research of Leob and Sourirajan. Alan Michaels fixed 1965 as the time when the first rudimentary UF membranes and devices first appeared on the market. UF membranes are made by immersion casting methods and are skinned and asymmetric. The initial commercial applications were related to protein concentration and the membranes were rated by the molecular weight of the protein that they would retain, i.e., the molecular weight cutoff rating of the membrane (MWCO).
While membrane ratings based on testing with proteins is still done, a common method uses non-protein macromolecules having a narrow molecular weight distribution, such as polysaccharides (Dextrans) or polyethylene glycols. See for example, A rejection profile test for ultrafiltration membranes and devices, BIOTECHNOLOGY 9 (1991) 941-943.
As membrane applications were developed in the 1960's and 1970's, UF membranes expanded to larger pore sizes and microporous membranes (MF) to smaller pores sizes. As this occurred, practitioners began to differentiate between the two types of membranes. It is interesting from a historical perspective that the earliest literature referred only to ultrafiltration. Both Kesting Synthetic Polymer Membranes A Structural Perspective, Robert E. Kesting, John Wiley & Sons 1985 and Lonsdale “The Growth of Membrane Technology”. K. Lonsdale, J. Membrane Sci. 10 (1982) 81 cite to Ferry's major review of 1936 in which ultrafiltration refers to both ultrafiltration and microfiltration membranes. Kesting states “The term ultrafiltration has changed its meaning over the years.” In fact, even in a 1982 review Pusch Synthetic Membranes—Preparation, Structure, and Application, W. Pusch and A. Watch Angew. Chem. Int. Ed. Engl. 21 (1982) 660 uses ultrafiltration to denote sieving membranes of from 0.005μ to 1μ. Kesting, in table 2.9 (pg 45) has UF as 10-1000 Angstroms, 0.01-0.1 microns, and MF as 1000-100.000 Angstroms, 0.1-10 microns.
A 1969 chart from Dorr-Oliver has microporous pore size ranging from 0.03μ to over 10μ, and UF ranging from 0.002μ to 10μ. A recent handbook chapter. Handbook of Separation techniques for Chemical Engineers—Third Edition, Section 2.1 Membrane Filtration, M. C. Porter, McGraw-Hill 1996 claims this “reflects confusion in the literature among MF, UF and RO.” In 1975 Porter Selecting the Right Membrane, M. C. Porter, Chem. Eng. Sci. 71 (1975) 55 proposed that UF cover the range from 0.001 to 0.02 microns, and MF from 0.02 to 10 microns. Lonsdale referred to this in Reference 2 and Porter uses this definition again in reference 4.
Cheryan Ultrafiltration Handbook, M. Cheryan, Technomic Publishing Co. Chapter 26—Introduction and Definitions (Ultrafiltration) S. S. Kulkarni et at Chapter 31—Definitions (Microfiltration) R. H. Davis 1986 has both Porter's ranges for UF and MF (uncited) and a chart that appears to be from the Dorr-Oliver chart. In Membrane Handbook, Davis, Van Nostrand and Reinhold DATE Davis gives MF as 0.02-10 microns, and Kulkarni et al describe UF as 10 to 1000 Angstroms, 0.001-0.1 microns. Another example of pore size ranges is from the Encyclopedia of Polymer Science and Engineering, Volume 9 pg 512, John Wiley and Sons 1987 which has UF as from 0.01 to 0.1 microns and MF as from 0.1 to 10 microns. Zeman Microfiltration and Ultrafiltration, L. Zeman and A. Zydney, Marcel Dekker, Inc 1996, p 13 has a chart in which UF ranges from 0.001 to 0.1 micron and MF from about 0.02 to 10 microns.
With respect to the present invention, we will define ultrafiltration membranes as compared to microporous membranes based on the definitions of the International Union of Pure and Applied Chemistry (IUPAC), “Terminology for membranes and membrane processes” published in Pure Appl. Chem., 1996, 68, 1479.
“72. microfiltration: pressure-driven membrane-based separation process in which particles and dissolved macromolecules larger than 0.1 μm are rejected”
“75. ultrafiltration: pressure-driven membrane-based separation process in which particles and dissolved macromolecules smaller than 0.1 μm and larger than about 2 nm are rejected.”
The definition for ultrafiltration membranes will be based on what they do, and how they do it. Ultrafiltration membranes are capable of concentrating or diafiltering soluble macromolecules that have a size in solution of less than about 0.1 micron and operating continuously in a tangential flow mode for extended periods of time, usually more than 4 hours and for up to 24 hours. Microporous membranes are capable of removing particles larger than 0.1 micron and being used in dead-end filtration applications. Microporous membranes generally allow soluble macromolecules to pass through the membrane.
Ultrafiltration membrane production methods by immersion casting are well known. A concise discussion is given in Microfiltration and Ultrafiltration: Principles and Applications Marcel Dekker (1996); L. J. Zeman and A. J. Zydney eds. These preparations are generally described to consist of the following steps: a) preparation of a specific and well controlled preparation of a polymer solution, b) casting the polymer solution in the form of a thin film onto a substrate, c) coagulating the resulting film of the polymer solution in a nonsolvent and d) optionally drying the ultrafiltration membrane.
The common form of ultrafiltration membranes is the asymmetric membrane, where the pore size of the membrane varies as a function of location within the thickness of the membrane. The most common asymmetric membrane has a gradient structure, in which pore size increases from one surface to the other. Asymmetric membranes are more prone to damage, since their retention characteristic is concentrated in a thin surface region or skin. A membrane skin is a thin dense surface penetrated by surface pores. It has been found, however, that increased productivity results from having the feed stream to be filtered contacting the larger pore surface, which acts to prefilter the stream and reduce membrane plugging.
Practitioners in the art of making ultrafiltration membranes, particularly asymmetric membranes, have found that membranes which contain large (relative to membrane pore size) hollow cavernous structures have inferior properties compared to membranes made without such hollow structures. These hollow structures are sometimes called “macrovoids”, although other terms are used in the art. Practitioners striving for membranes of very high retention efficiency prefer to make membranes without such hollow structures.
Perhaps the most direct variation of the single layer structure is a multilayered unbonded laminate. While laminates can be made from layers of the same or different membranes, they have drawbacks. Each layer has to be made in a separate manufacturing process, increasing cost and reducing manufacturing efficiency. It is difficult to manufacture and handle very thin membranes, less than say 20 microns, because they deform and wrinkle easily. This adds to the inefficiency of producing a final product with thin layers. Unbonded laminates can also come apart during fabrication into a final filter device, such as a pleated filter, which will cause flow and concentration non-uniformities. Other methods of forming multilayered porous membrane structures are known. U.S. Pat. No. 4,824,568 describes a composite ultrafiltration membrane made by casting a thin ultrafiltration membrane onto a preformed microporous membrane. U.S. Pat. No. 5,228,994 describes a method for coating a microporous substrate with a second microporous layer thereby forming a two layer composite microporous membrane. These processes require two separate membrane forming steps, forming one on top of the other preformed membrane and are restricted by the viscosities of the polymer solutions that can be used in the process to prevent excessive penetration of casting solution into the pores of the preformed substrate.
In U.S. Pat. No. 5,620,790, a method of making a microporous membrane is described wherein the membrane is made by pouring out a first layer on a support of polymeric material onto a substrate and subsequently pouring out one or more further layers of a solution of polymeric material onto the first layer prior to the occurrence of turbidity in each successively immediate preceding layer, the viscosity of each immediately successive layer of a solution of polymeric material having been the same or less than that of the preceding layer. US Patent Application 20030217965, directed to microporous membranes, provides for a method of producing an integral multilayered porous membrane by simultaneously co-casting a plurality of polymer solutions onto a support to form a multilayered liquid sheet and immersing the sheet into a liquid coagulation bath to effect phase separation and form a porous membrane. U.S. Pat. No. 6,706,184 discloses a process for forming a continuous, unsupported, multizone phase inversion microporous membrane having at least two zones comprised of the acts of: operatively positioning at least one dope applying apparatus, having at least two polymer dope feed slots, relative to a continuous moving coating surface; applying polymer dopes from each of the dope feed slots onto the continuously moving coating surface so as to create a multiple layer polymer dope coating on the coating surface; subjecting the multiple dope zone layer to contact with a phase inversion producing environment so as to form a wet multizone phase inversion microporous membrane; and then washing and drying the membrane. In these structures, each layer or zone is a microporous membrane. US Patent Application 20040023017 describes a multilayer microporous membrane containing a thermoplastic resin, comprising a coarse structure layer with a higher open pore ratio and a fine structure layer with a lower open pore ratio, wherein said coarse structure layer is present at least in one membrane surface having a thickness of not less than 5.0μ, a thickness of said fine structure layer is not less than 50% of the whole membrane thickness, and said coarse structure layer and said fine structure layer are formed in one-piece. The fine structure is not skinned. This structure is formed from a single solution.